1. Field of the Invention
This invention relates to digital television signal processing and more particularly to methods and apparatuses for transmitting digital television signals in an error-correction format.
2. Description of the Prior Art
There are two main approaches to dealing with errors in digital television signals. The first approach is correction, which involves the production and use of additional data signals purely for the purpose of error detection and correction, these additional data signals otherwise being redundant. The second approach is concealment, which involves additional data signals for detection of errors only, followed by the reconstruction of corrupted data signals using available uncorrupted data signals and relying for accuracy on the strong correlation that exists in a television signal not only in the horizontal and vertical dimensions but also temporally.
Clearly, error correction is potentially the better method of dealing with errors. However, there are difficulties. Firstly, for a reasonable level of protection from errors which escape correction, a high level of redundancy is required. Incidentally, the redundancy level is typically expressed in percentage terms, that is, the number of redundant bits as a percentage of the total number of bits. Secondly, there is some risk of wrong correction, that is, where a valid bit is wrongly changed to the other binary value. Error correction codes for use with binary data are of course well known, but the error rates which can occur in a digital television signal, due for example to drop-out on reproducing from a magnetic tape, are very high, and in these circumstances some of the known error correction codes can actually have the effect of increasing the number of errors.
A digital television signal is normally organised into blocks of data, each block comprising both information bits and redundant bits. Error detection is a means by which the redundant bits are used to indicate the occurrence of one or more errors within a given data block. The indication can only be one of two possibilities; that is, the block has errors or it does not have errors. All usable methods of error detection will detect one error in a block, and the performance of the method is defined by the detection capability for multiple errors. In a video tape recorder (VTR), three forms of multiple error occur, these being:
(i) errors caused by noise PA1 (ii) interchannel cross-talk, which is sometimes caused by poor tracking when a VTR is operating in the normal reproduction mode, but it is particularly liable to occur when a VTR is operating in a special mode, such as slow motion, fast motion, stop motion or reverse motion; and PA1 (iii) drop-out, which may cause total loss of reproduced data for a considerable length of time.
Moreover, particularly with digital VTRs for broadcast use, it is common for a video signal to pass through many generations of recording and reproduction. This demands a superlative first generation picture. For this reason there is still a strong demand for better methods of and apparatuses for error correction. And while it is recognised that any correction method will fail from time to time and then concealment will be required, this should preferably only occur in extreme circumstances.